fix flaky addrman_is_terrible test

Summary: AddrInfo::IsTerrible takes an optional now argument defaulting to AdjustedTime(). Set the mocktime to ensure the adjusted time cannot change before the end of the test.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15306